A voltage across a capacitor connected to a dc side of a converter (referred to as a capacitor voltage) includes a ripple component (rectification ripples). Given that the frequency of electric power from a source is f0, the frequency of the ripple component is 2×f0 when the source is single-phase ac, and 6×f0 when the source is three-phase ac.
Because the capacitor voltage is inputted into an inverter, the ac voltage outputted from the inverter also includes the ripple component. When the output frequency of the inverter becomes close to the ripple frequency of the capacitor voltage, due to the differential frequency between them, a beat phenomenon occurs in which ac electric power as output from the inverter and a torque of the motor driven by the inverter fluctuate. The reason for the beat-phenomenon occurrence is attributed to inverter output-voltage amplitude characteristics in which the positive-side characteristic and the negative-side characteristic are not balanced during a cycle.
A method for preventing the beat phenomenon has been considered, in which, in a case of a ripple component being included in the input voltage of the inverter, the operation frequency of the inverter is varied in such a way that the voltage-time integration value for a positive-side half cycle and that for a negative-side half cycle become equal to each other. In this method, the reason for varying the operation frequency of the inverter is that the beat phenomenon occurs at a frequency which is included in a constant-voltage variable-frequency region where the voltage cannot be controlled (for example, referred to as Patent Document 1).
A method for removing or controlling the beat has been also considered in a vector control, in which a phase of a voltage vector is controlled (for example, referred to as Patent Document 2).
By detecting the ripple component of the capacitor voltage, the phase of the detected ripple component is shifted forward, and a voltage value is obtained by summing with a dc component of the capacitor voltage. Thus, a method for controlling a modulation factor using the voltage value has been also considered (for example, referred to as Patent Document 3).